Modernise build structure
Ben Avison authored
Detail:
* Rewrite `Makefile` to use shared makefile fragments
* No longer requires `-E` switch to `amu`
* Consolidate `.gitignore` files

Admin:
  Requires BuildSys-7_50 and RISC_OSLib-6_08.

Version 1.75. Tagged as 'EditApp-1_75'
a82638c1